This invention relates to phase-locked loops.
Radio transceivers gain flexibility at reduced cost by the use of synthesizers as sources of the radio-frequency signals for transmission and for receiving. Such a circuit allows the use of a single frequency-determining element to enable a selection of many channels. This represents a significant saving over the use of multiple crystals, one for each channel that is to be used. When a synthesizer is used in the typical installation for land mobile or fixed station radio communication, using voice signals, a phase-locked loop may be used to generate a frequency that is deviated at an audio rate to produce a modulated signal. When the modulating signal is voice, there is no problem in controlling the deviation of the phase-locked loop and thus the synthesized frequency. When the modulating signal is voice which has a zero average value, the resultant deviation of the synthesizer output frequency around the carrier frequency is a very close representation of the modulating signal.
The situation changes, however, when it is desired to use frequency-shift keying to send digital data in a circuit that uses a synthesizer. There is a significant probability that the data will have a non-zero average value. In other words, the data may have a DC component. The operation of a synthesizer is such that it will not maintain the DC component of a signal, but will cause the average value ( the DC component) of the signal to approach asymptotically the at-rest carrier frequency. The long-term effect of this action is to generate unequal frequency deviation of the signal around the carrier value.